The Massachusetts Bar Association called Thursday for an independent investigation into the reliability of breathalyzer tests used to prosecute people suspected of drunken driving and urged a moratorium on introducing the tests as evidence until concerns are resolved.

Martin W. Healy, chief legal counsel for the association, said the state attorney general should appoint an investigator without ties to law enforcement to lead the inquiry.

The call from the state’s leading confederation of attorneys emerged as prosecutors, defense attorneys, and the State Police scoured case files for evidence that breathalyzer readings were improperly calibrated. The tests can play an important role in prosecutions, often persuading motorists who have been observed driving erratically to plead guilty, defense attorneys said.

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“Drunk driving is a very serious issue in Massachusetts, and the Mass. Bar Association recognizes that,” Healy said. “But people’s constitutional rights are also important. We don’t want to see people convicted and taking plea agreements based on faulty evidence.”

State Police have already opened a review of the concerns regarding breathalyzer results, with results expected next week. Agency spokesman David Procopio said State Police believe officers might have accepted test results from breathalyzer machines that were not properly calibrated, although the review will determine whether that is what really happened.

Meantime, a growing roster of prosecutors have decided to suspend use of the breathalyzer test results as evidence. The Essex district attorney was notified in mid-March by State Police about a possible issue with breathalyzers.

On Thursday, district attorneys in Suffolk, Worcester, Norfolk, and Plymouth counties, as well as the Northwestern district attorney, announced they were suspending use of the results. They joined DAs in Middlesex and Essex counties and the Cape and Islands who had already announced halts.

Berkshire District Attorney David F. Capeless said his office was not suspending use of breathalyzer results. His prosecutors are doing their own review of cases but felt comfortable that they know which results were compromised.

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“We’re doing it on a case-by-case basis,” he said.

Representatives from other DA offices in Massachusetts did not respond to requests for comment.

A spokesman for the Massachusetts Executive Office of Public Safety said Wednesday that he was unable to confirm how many cases might be affected. An official with knowledge of the review said authorities believe the number to be about 69, out of roughly 6,000 tests performed statewide.

Healy, of the state Bar Association, said the call for an independent investigation was not meant to impugn the State Police investigation but to add an extra layer of scrutiny.

“Some in the defense community would question the findings of a police department or police agency regarding the very device that they use to prosecute and convict individuals for drunk driving,” he said.

A spokesman for the attorney general’s office said it was working with other agencies on what should be done but did not comment directly on the call for an additional investigation.

“Any time there is a question about the validity of evidence used in a criminal proceeding, the attorney general is concerned,” spokesman Christopher Loh said. “Our priority is to ensure that the integrity of the system is upheld, and we are speaking with [the Executive Office of Public Safety and Security], the district attorneys, the Massachusetts Bar Association, and defense bar, to determine the best steps needed to help accomplish that.”

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Felix Browne, spokesman for the state Office of Public Safety, issued a brief statement Thursday without responding to questions about when the problem with breathalyzer results might have begun, how it was discovered, or the possibility of an additional investigation.

“The Executive Office of Public Safety and Security became aware of a small number of breathalyzer readings that did not comply with the stringent requirements of Massachusetts regulations governing breath-test procedures,” he said. “As a result, the office is performing a review of these tests, which we expect to complete early next week.”

First, spokesman Jake Wark said, prosecutors will seek to have all breathalyzers in Boston, Chelsea, Revere, and Winthrop tested and calibrated. Second, they will undertake a review of drunk-driving convictions that used breathalyzer evidence since the current machines were introduced.

Only two cases have been identified in Suffolk as being possibly problematic, he said. In those cases, the devices calibrated within the manufacturer’s guidelines but outside the narrower range imposed by state regulations, he said.

“While we have no reason to believe that the very small number of affected Suffolk cases will increase, we have every reason to seek certainty that past convictions are based on reliable evidence,” Wark said.

Breathalyzers are not the only evidence used in drunken driving cases, according to attorneys and law enforcement officials. Police officers and state troopers administer field sobriety tests and observe a person’s behavior, recording tip-offs such as erratic driving or causing a car accident, slurred speech, glassy eyes, trouble walking, and the odor of alcohol. Often, Procopio said, the breathalyzer is the last piece of evidence collected.

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“People can refuse the breathalyzer and still be arrested,” Procopio said.

Still, the presence of a breathalyzer can persuade defendants to plead guilty rather than fight their case.

“In an OUI case, the breath test is the ultimate piece of evidence,” said John B. Seed, a defense attorney with a concentration in operating under the influence defense who said he finds properly calibrated breathalyzer results to be fairly reliable. “When you don’t have that, all you have left are whatever observations the officer had prior to placing the person under arrest.

“Many people who don’t take a breath test end up fighting the case,” he said, “because that evidence can be cross-examined and discredited to a certain degree.”

Some attorneys, however, said that while much emphasis is placed on breathalyzers, the current questions about calibration are not new.

“Some Massachusetts attorneys view breathalyzer machines as akin to polygraph machines,” Healy said. “They’re well recognized for improperly working and for misreads.”

Attorney J. Albert Johnson, a nationally known trial lawyer, called the use of the breathalyzer “morally wrong and legally indefensible.”

“It has never worked; it will never work,” Johnson said. “No machine manufactured by man can transform breath into blood alcohol.”

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In his statement Wednesday, Browne, of the state public safety agency, called breathalyzers, when properly used and maintained, “one of the most accurate and reliable tools we have to identify and investigate drunk drivers.”

With the investigation underway, some expressed concern that the number of cases affected could grow.

“I’m not buying into the fact that it’s limited to 69 cases,” said Stephen J. Weymouth, a Boston defense lawyer, who noted that an Essex County case that officials have said might be affected dates to late 2011. “It’s a machine that’s used fairly regularly and fairly often. It’s just the beginning of the review.”

Globe correspondent Rebecca Fiore contributed to this report. Evan Allen can be reached at evan.allen@globe.com.